1. Field of the Invention
The present invention relates to a rotary stencil printer, and more particularly to a construction for operating an inner press roller of a rotary stencil printer in which a part of a cylindrical wall made of an ink permeable flexible sheet material of a printing drum is pressed radially outward by the inner press roller from the inside thereof during a printing operation, and a method of controlling the operation of the inner press roller.
2. Description of the Prior Art PA1 a frame body; PA1 a printing drum having a circumferential wall made of an ink permeable flexible sheet material and supported by the frame body to be rotatable on a central axis thereof; PA1 a back press roller having a cylindrical outer surface and supported by the frame body to be rotatable on a central axis thereof parallel with the central axis of the printing drum so as to define a nip region between the cylindrical outer surface thereof and a cylindrical outer surface of the printing drum for nipping and transferring a print sheet therethrough; PA1 a first arm means supported by the frame body to swing about a pivot axis thereof parallel with and distant from the central axis of the printing drum; PA1 an inner press roller supported by said first arm means to be rotatable on a central axis thereof parallel with and distant from the pivot axis of said first arm means to contact a part of the circumferential wall of the printing drum at a radially inside surface thereof so as to selectively press said part radially outward of the printing drum toward the back press roller when biased in a radially outward direction of the printing drum; PA1 a first gear wheel adapted to rotate on the central axis of the printing drum in synchronization with the printing drum; PA1 a second arm means adapted to swing about the central axis of the printing drum; PA1 a second gear wheel supported by said second arm means to be rotatable about a central axis thereof and meshing with said first gear wheel; PA1 a third gear wheel adapted to rotate about the central axis of the inner press roller together therewith and meshing with said second gear wheel so that the inner press roller is rotated in synchronization with the printing drum through a gear train of said first, second and third gear wheels when the printing drum is driven to rotate on the central axis thereof; and PA1 a brake means for braking rotation of at least one of said second and third gear wheels against the corresponding support arm means so that a reaction torque generated by an actuation of the brake means generates a force to bias the inner press roller in said radially outward direction of the printing drum. PA1 In the rotary stencil printer of the above-mentioned basic construction, the central axis of said second gear wheel may be located on a first side of a phantom plane extended between the central axes of the printing drum and the inner press roller opposite to a second side thereof at which the pivot axis of said first arm means is located. PA1 The rotary stencil printer of the above-mentioned basic construction may further comprise a means for restricting a distance between the central axes of said second and third gear wheels from increasing beyond a predetermined distance to ensure a designed meshing therebetween.
As a type of rotary stencil printer there has been proposed in Japanese Patent Laid-open Publication 1-204781 filed by the same assignee as the present application a rotary stencil printer having a basic construction which comprises a printing drum having an ink permeable flexible circumferential wall, a back press roller opposing the printing drum at an outside thereof in close proximity thereto, and an inner press roller for selectively pressing a part of the circumferential wall of the printing drum from the inside thereof radially outward toward the back press roller, wherein the inner press roller is supported to be rotatable on a central axis thereof by an arm means adapted to swing around a pivot axis parallel with and distant from a central axis of the printing drum, so that when the arm means is positioned at a first swing position around the pivot axis, the inner press roller is just in contact with or apart from the circumferential wall of the printing drum, while when the arm means is at a second swing position around the pivot axis, the inner press roller presses a part of the circumferential wall of the printing drum radially outward toward the back press roller, so that when a stencil is mounted around the circumferential wall of the printing drum and the printing drum is rotated in synchronization with the back press roller in mutually opposite rotational directions with the part of the circumferential wall of the printing drum being pressed radially outward by the inner press roller toward the back press roller, a print sheet fed into a nipping region defined between the opposing portions of the printing drum and the back press roller is applied with a print image according to the perforations of the stencil by ink supplied to the inside of the printing drum. Further, there has been proposed in Japanese Patent Laid-open Publication 2-225078 filed by the same assignee as the present application a rotary stencil printer having the above-mentioned basic construction and improved with respect to the ink permeable flexible circumferential wall of the printing drum.
Further, there has been proposed in Japanese Patent Laid-open Publication 3-254984 filed by the same assignee as the present application a rotary stencil printer which has the above-mentioned basic construction and further incorporates a gear train for synchronizing the rotation of the inner press roller with the rotation of the printing drum, so that the inner press roller is positively rotated, instead of being rotated by following the rotation of the printing drum due a frictional contact therewith via a viscous ink layer, at a distinctly predetermined rotation ratio relative to the rotation of the printing drum, thereby stabilizing the squeezing action applied to the circumferential wall of the printing drum by the inner press roller, and also biasing the inner press roller radially outward against the circumferential wall of the printing drum by a force transmitted to the inner press roller through the gear train. In this rotary stencil printer, however, although the stability of the squeezing action is highly improved as the rotation of the inner press roller is definitely synchronized to the rotation of the printing drum by the gear train, the force transmitted to the inner press roller through the gear train is not necessarily stabilized.